VASCULAR SMOOTH-MUSCLE RELAXATION BY ALPHA-ADRENOCEPTOR BLOCKING ACTION OF DOBUTAMINE IN ISOLATED RABBIT AORTA

We investigated the mechanism of vascular relaxation produced by dobut amine, a positive inotropic agent with beta(1)-adrenergic action. Dobu tamine concentration-dependently (10 nM-10 mu M) relaxed ring segments of rabbit aorta partially precontracted with 1 mu M phenylephrine (PE ) but did not relax those precontracted with 40 mM K+ or 5 mu M prosta glandin F-2 alpha (PGF(2 alpha)). The relaxation was not completely in hibited by pretreatment with 10 mu M propranolol. Dobutamine did not s ignificantly increase tissue cyclic AMP levels concomitantly with rela xation as does isoproterenol (ISO) in rabbit aorta, Dobutamine produce d a parallel rightward shift in concentration-response curves to PE. T he Schild plot analysis resulted in a linear regression of a slope of 1.077 +/- 0.077, which was not significantly different from unity. The pA(2) value of dobutamine as compared with PE in rabbit aorta was 6.8 1 +/- 0.03. Dobutamine causes arterial dilatation mediated not only th rough a beta-adrenergic action but also through an alpha-adrenergic bl ocking action in rabbit aorta.